The present invention relates to a method of cooling a moving strand of hot material in which the strand is passed through a plurality of cooling tubes and is water cooled while it passes through the tubes. The strand may be, for instance, hot-rolled rod or an extrusion coming from an extruder and passing through a cooling apparatus at speeds above 35 meters/second.
When rolling rod, such cooling tubes, known as a water-cooling section, are positioned directly after the last or finishing roll stand of the rod mill in order to cool the rod down as quickly as possible to the temperature desired for the proper heat treatment of the rod, before the rod reaches the reel. The cooling should not be too rapid, otherwise excessive temperature gradients would be set up between the core and the surface of the rod, and the difference in temperatures could lead to different transformations and non-uniform structure across the section of the rod; in addition, the temperature should not fall below about 500.degree. C in order to avoid the risk of martensite formation.
One way of carrying out the method is to pass a flow of water through the cooling tubes, but a disadvantage of this method is that the leading end or tip of the rod encounters a high braking resistance due to the dynamic pressure of the water in the cooling tube; the leading end buckles slightly and the rod is thrust against the wall of the tube, leading to increased friction. In this way, the rod sometimes breaks out of the water-cooling section.
In order to avoid the braking action of the water on the leading end of the rod, it is possible to admit water to the cooling tube only after the leading end of the rod has been introduced; however, with the current high rod speeds, this method suffers from the disadvantage that the first fifty to a hundred meters of each rod are cooled in an uncontrolled manner and must be rejected because they have a non-uniform structure and have a thicker layer of mill scale.